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Line Width Measurement Technique Using Through-Focus Optical Images



Ravikiran Attota, Richard M. Silver, Ronald G. Dixson


We present a detailed experimental study of a new through-focus technique to measure line width (CD) with nanometer sensitivity using a bright field optical microscope. This method relies on analyzing intensity gradients in optical images at different focus positions, here defined as the focus metric signature. The contrast of an optical image of a structured target, where a particular structure is repeated several times, varies greatly as it is moved through-focus if the spacing between the structures is such that the scattered field from the features interferes. Complex, distinguishable through-focus optical response occurs under this condition giving rise to the formation of several cyclic high and low contrast images. As a result it exhibits several focus metric signature peaks as opposed to a single focus metric peak for structures nearly isolated. This complex optical behavior is very sensitive to the dimensions of the target geometry. By appropriately analyzing the through-focus optical image, information can be obtained regarding the target. In the present work an array of lines is used as a structured target. Line width measurements were made by using experimental through-focus optical data obtained using a bright field microscope and simulated optical data. The optical results are compared with reference metrology tools such as critical dimension atomic force microscope and critical dimension scanning electron microscope.
Applied Optics


CD metrology, focus metric signature, optical critical dimension, through-focus images


Attota, R. , Silver, R. and Dixson, R. (2008), Line Width Measurement Technique Using Through-Focus Optical Images, Applied Optics, [online], (Accessed April 18, 2024)
Created January 1, 2008, Updated February 19, 2017